CSEDI: Geochemical Evolution of the Earth's Mantle Constrained by Observations and Dynamical Modeling

这个项目旨在推进关于地球内部在地质时期是如何变化的科学知识，由于构造板块下沉到内部和热浮力物质从地球深处上升引起的非常缓慢的运动，所有这些都是为了混合地球的地幔。这些对流运动有可能将构造板块从地表一直移动到核心-地幔边界（CMB），并从CMB返回物质，在那里它可以促成近地表融化，从而在海洋中脊和海洋热点（如夏威夷和其他地区）产生火山。研究人员将使用高性能计算机模拟这些对流运动来研究地球内部的流动，以及在地球内部深处的高压下，这种流动是如何被矿物变化所改变的；当地幔在地表融化时，当板块下沉并混合到地球内部时，当大陆的沉积物被这些过程带入地球内部时，记录地球化学变化的数以百万计的被动示踪粒子将增强计算模型。示踪粒子记录的地球化学变化将与火山的地球化学数据进行比较，以测试地球深处的流动是一直持续到地核，还是内部流动是作为单独的层流动，它们之间的混合程度最低。从这个项目中获得的知识将提供有关地球表面构造运动如何影响内部混合的信息，以及这种混合如何反过来改变从内部到大陆的各种元素的流动，从而提供有关大陆在地球地质历史上如何生长和侵蚀的信息。在这个项目中，研究人员将专注于用一系列高分辨率的数值对流模型来记录地幔的地球化学演化，这些模型检查了当使用实际的可压缩状态方程来描述地幔的物理特性时，地幔分层、分层和对流隔离的程度。可压缩地幔的使用将使我们能够以严格的方式模拟矿物相变。当与地幔粘度、电导率和热膨胀的p - t应变率相关的实验室数据相结合时，该模型的这些特征将产生对地幔分层程度的最物理真实的描述，这种分层可能是由于与已知相边界相关的深度相关的密度和粘度而发生的。模型系列将进一步局限于那些与主要地球物理和地球化学观测（对流活力、地幔温度、热流、板块速度、内部加热速率、大气和大陆地壳的成分和质量等）的已知范围相匹配的模型。模型空间的地球化学将通过嵌入地幔流中的数百万个活跃的数值示踪剂来计算，这些示踪剂将记录它们经历的微量元素分馏和相关的同位素演化，当它们的演化被发生在模型表面的熔化、脱气和大陆提取事件打断时。该团队将明确模拟对流地幔、大陆地壳和大气中亲石和稀有气体同位素系统的演化同位素地球化学，并将模型输出与地球化学数据进行定量比较，以测试我们模型的可行性。

This project seeks to advance scientific knowledge about how the Earth's interior has changed over geologic time, due to the very slow motions caused by the sinking of tectonic plates into the interior and the rise of hot buoyant material from the deep Earth, all of which work to mix the Earth's mantle. These convective motions have the potential to move tectonic plates from the surface all the way to the core-mantle boundary (CMB), and to return material from the CMB where it can contribute to near-surface melting that produces volcanoes at mid-ocean ridges and at oceanic hotspots (such as Hawaii and others). The investigators will use high-performance computer simulations of these convective motions to study the flow of the Earth's interior and how this flow is modified by changes in mineralogy that occur at the high pressures of the Earth's deep interior; the computational models will be enhanced by the inclusion of millions of passive tracer particles that record changes in geochemistry that occur when the mantle melts at the surface, when plates sink and mix into the Earth's interior, and when sediment eroded from the continents is brought into the Earth's interior by these processes. The geochemical changes recorded by the tracer particles will be compared with geochemical data from volcanoes to test whether the flow in the deep Earth continues all the way to the core, or whether the interior flows as separate layers with minimal mixing between them. The knowledge gained from this project will provide information on how the tectonic motions at the surface of the Earth affect mixing in the interior, and how this mixing in turn has altered the flow of various elements from the interior to the continents thus providing information on how the continents have grown and eroded over Earth's geologic history.In this project the investigators will focus on documenting the geochemical evolution of the mantle in a series of high-resolution numerical convection models that examine the degree of mantle layering, stratification, and convective isolation that occur when realistic compressible equations of state are used to describe the physical properties of the mantle. The use of a compressible mantle will allow us to model mineralogical phase transitions in a rigorous way. When combined with laboratory data on the P-T-strain rate dependence of mantle viscosity, conductivity and thermal expansion, these features of the model will produce the most physically realistic description of the extent of mantle layering that can occur due to depth-dependent density and viscosity associated with known phase boundaries. The model series will be further constrained to only those models that match the known ranges in major geophysical and geochemical observations (convective vigor, mantle temperature, heat flow, plate velocities, rate of internal heating, composition and mass of the atmosphere and continental crust, etc.). The geochemistry of the model space will be calculated from millions of active numerical tracers embedded in the mantle flow, that will each record the trace element fractionation and associated isotopic evolution they experience when their evolution is punctuated by melting, degassing, and continental extraction events that occur at the surface of the model. The team will model explicitly the evolving isotope geochemistry of both lithophile and noble gas isotope systems in the convecting mantle, continental crust and atmosphere, and quantitatively compare the model output with geochemical data to test the viability of our models.